A Mannesmann mill process has been widely used as a method for manufacturing seamless steel pipes using hot processing in the past. This is a method for manufacturing a seamless steel pipe having specified dimensions by firstly performing piercing on a round shape steel (hereinafter, called a billet) which has been heated up to a specified temperature using a piercing mill in order to make a hollow piece (hereinafter, called a hollow), by decreasing the thickness of the hollow using a main rolling mill such as an elongator, a plug mill or a mandrel mill, by further reheating the hollow as needed and then by mainly reducing the outer diameter of the hollow using a reducing mill or a sizing mill.
As for the piercing mill mentioned above, there are various kinds of piercing mills, and common examples of piercing mills include a so-called Mannesmann piercer consisting of two barrel type rolls, a plug and two guide shoes, a so-called 3-roll piercer consisting of three barrel type rolls and a plug, and a so-called press rolling piercer consisting of two grooved rolls and a plug.
In the piercing process mentioned above, since a plug is constantly exposed to a high temperature and a high load due to ceaseless contact with a heated billet or a hollow, the plug tends to undergo wear or deformation due to an elevated temperature. Therefore, a scale film having a thickness of several tens of μm to several hundreds of μm may be formed on the surface of the plug by performing scale handling on the plug at a high temperature of 900° C. to 1000° C. in order to prevent wear damage. For example, Patent Literature 1 discloses a technique in which iron oxide scale mainly containing magnetite is formed on a surface of a plug having a base metal composed of an iron-base alloy by performing a heat treatment on the plug. Since such oxidized scale prevents metallic contact between the metal of a rolled material and the metal constituting the plug by being present as a nonmetallic coating between the metals when hot rolling is performed, seizure and deposition are prevented and the amount of wear is decreased, which results in there being effects of protecting the plug and increasing the life of the plug. In the case where a rolled material is a high alloy containing a large amount of Cr, since there is a decrease in tool life due to frequent metallic contact between the material and a tool such as a plug because only a very small amount of surface scale is generated due to the nature of the material when the material is heated, such a technique in which oxidized scale is artificially formed on the surface of a tool is particularly effective.
However, in the case where a rolled material is high-alloy steel such as steel containing 12 mass % or more of Cr, since the number of rolled materials which can be rolled with one plug is only about 10 at most even using the technique described above, a further increase in tool life is required.
The reason why plug life is insufficient in the case where a rolled material is high-alloy steel containing 12 mass % or more of Cr is that, since the high-temperature strength of a plug composed of an iron-base alloy is comparatively low because the strength of the rolled material is high during hot processing, deformation such as the crush of the plug tip or the gouge of the surface of the plug occurs due to a contact load even though the surface of the plug is protected with oxidized scale, which results in the surface scale layer being broken and defects such as seizure occurring.
Therefore, in order to increase the life of a plug for the piercing of a seamless steel pipe in the case where high-alloy steel described above is rolled, methods such as one in which the whole or tip of a plug is composed of ceramic (Patent Literature 2) or a molybdenum alloy having excellent high-temperature strength (Patent Literature 3), one in which a plug tip is coated with a cobalt-base alloy having a high high-temperature strength by performing powder overlaying welding (Patent Literature 4) and one in which a plug is composed of or coated with a Nb alloy (Patent Literature 5) have been proposed. Moreover, Patent Literature 6 proposes a tool in which a metal-carbide compound film having a matrix metal composed of a cobalt-base alloy or a nickel-base alloy with niobium carbide particles being dispersed in the matrix is formed on the surface of the tool and in which a ferrous oxide film is formed on the outermost surface of the tool.